1. Field of the Invention
The present invention relates to a device for precisely establishing in the laboratory the curve of the resistivity index of a solid sample independently of the capillary pressure curve, suited for high-frequency measurement.
2. Description of the Prior Art
Measurement of the resistivity index of small cores is necessary to obtain a precise estimation of the water saturation from log data obtained for example by means of the measurement while drilling (MWD) technique.
French Patent 2,781,573 and U.S. Pat. No. 5,979,223, filed by the assignee notably describe methods and devices intended for continuous measurement of the resistivity index curve of a solid sample initially saturated by a first wetting fluid, such as a geologic sample, independently of the capillary pressure curve. The porous solid sample is contained in a sealed sheath placed in an elongate containment cell between two terminal parts. Channels provided through the two terminal parts communicate with an injection system which injects a second, non-wetting fluid into the sample at a first end of the cell and drains the first fluid from the cell at the opposite end, through a semipermeable membrane permeable to the first fluid. The sample is contained in a sheath and subjected to a radial pressure by injection of oil under pressure into the annular space between the body of the cell and the sheath. A membrane, wettable only by the second fluid, is interposed between the sample and the first end of the cell for re-imbibition operations.
Electrodes interposed between the sample and a sheath apply an electric current and detect potential differences that appear between distinct points in response to the application of an electric current. The electrodes are connected to a device measuring the complex impedance of the sample. The longitudinal extension of the electrodes is relatively great in relation to the length of the sample so that the largest possible part of the volume of the sample is involved in the impedance measurement while avoiding short-circuits through the ends of the sample which are likely to distort the measurements.
One or more injection pressure stages are applied and the continuous variations of the resistivity index as a function of the mean saturation variation are measured without waiting for the capillary equilibria to be established.
The annular space between the sheath and the external wall of the cell is under high pressure and the electric conductors connecting the electrodes to the measuring device run through the external wall of the cell through sealed bushings (glass bead connectors for example).
Studies show that the resistivity index of porous rocks varies substantially with the frequency. As logging sondes measure the electric resistance during crossing of formations often at very high frequencies, the sondes must be able to work with precision in the same frequency range in order to really compare the measurements obtained by means of the well tools to the resistivity index measurements obtained in the laboratory by means of the cells.
The results obtained with the previous cells are satisfactory when the frequency range of the applied electric currents remains within a limit of several to ten KHz. The results lose a lot of significance when the impedance measurements are carried out at much higher frequencies ranging for example between 1 MHz and 10 MHz. At such frequencies, shielded cables of constant impedance must of course be used. Continuous connection of the electrodes to the measuring device by means of shielded cables is difficult to achieve because of sealing problems. If a conventional connector of glass bead type is used, this leads to a break in the continuity of the shielding. This discontinuity, which would have no notable effect at low frequencies, is the source of parasitic reflections and of a significant attenuation of the signals at high frequencies.
The device according to the invention allows connection, by means of a shielded cable, to at least one electrode to a measuring device, located on either side of a wall separating an enclosure under pressure from the outside environment. The device comprises at least one rigid protector sleeve made of an insulating material that tightly runs through the wall and extends to the immediate vicinity of the electrode, wherein the shielded cable is passed. The rigid sleeve contains a tube made of a conducting material that is in electric contact with the shield of the cable and is rigidly and tightly associated with a connection means connecting electrically the core of the shielded cable to the electrode.
The electric connection means comprises, for example, a plug connected to the electrode, with a baseplate that is rigidly and tightly fastened in a cavity of the sleeve and electrically connected to the core of the shielded cable.
In order to account for a possible motion of the electrode, the plug is engaged in a hollow of the electrode and maintains electric contact with the electrode in which the plug moves.
In an embodiment the device of the invention comprises an electric connector associated with the rigid sleeve outside the wall for connection of a shielded wire connected to the measuring device, a shielded cable element within the rigid sleeve whose core is connected to the connection means, and the shield is electrically connected to the conducting tube that extends to the inside of the rigid sleeve as far as the zone where the core is connected to the connection means.
The wall is, for example, the wall of the body of a cell which measures of the variations of the resistivity index of a porous solid sample embedded in a sheath and is subjected to a radial pressure by injection of a liquid under pressure into the body of the cell, these variations resulting from operations of forced displacement of a first fluid out of the sample by injection of a second fluid, one of the two fluids being electricity conducting, unlike the other one which is not electrically conductive, by means of electrodes arranged between the sample and the sheath and provided each with an extension running through the sheath, each rigid sleeve running through the wall of the cell body and extending substantially to the sheath.
The measuring system according to the invention comprises an elongate containment cell for a sample in a sheath, means for injecting a liquid under pressure into the body of the cell so as to exert a radial pressure on the sample, electrodes arranged between the sample and the sheath, which apply an electric current and detect potential differences appearing between distinct points of the sample in response to the application of the electric current. The electrodes are each provided with an extension running through the sheath and are connected to a device measuring the impedance of the sample, outside the cell body, a first semipermeable filter permeable to the first fluid and contacting with a first end of the sample, and injecting means for injection under pressure a second fluid through a second end of the sample. The system comprises connection devices for connecting the various electrodes to the measuring device by means of shielded cables and each connection device comprises at least one rigid protector sleeve made of an insulating material that tightly runs through the wall and extends to the immediate vicinity of the electrode, wherein the shielded cable is passed, the rigid sleeve containing a tube made of a conducting material in electric contact with the shield of the cable and being rigidly and tightly associated with a connection means connecting electrically the core of the shielded cable to the electrode.
The electrodes preferably have a relatively great longitudinal extension in relation to the length of the sample (between xc2xc and xc2xe of the length of the sample and preferably of the order of xc2xd) but are smaller than the length, so that the largest possible part of the volume of the sample is involved in the impedance measurements while avoiding short-circuits through the ends of the sample.
The connection device defined above is advantageous in that it allows a shielded wire to run tightly through a wall without any discontinuity of the core and of the shield of the cable which are likely to affect the signals transmitted, in a frequency range that can reach multiples of ten MHz.
The measuring system, with its connection device(s) as defined above, is particularly advantageous in that it allows:
establishing a very precise curve of the continuous resistivity index during drainage in a short time (about 2 days for a typical 100 mD sandstone whereas the typical time required using the continuous injection technique is often of the order of two weeks);
the negligible incidence of non-uniform saturation profiles during measurement. The above advantages are the result of the combination of three factors: (i) the radial resistivity measuring technique, (ii) the presence of semipermeable filters at the outlet, (iii) the total volume of the core is analyzed by means of electric measurements (which is verified when the diameter of the core is greater than its length); and
providing precise resistivity index measurements in a very wide frequency range up to frequencies of the order of multiples of ten MHz.